Digital cameras employing complementary metal-oxide semiconductors (CMOS) image sensors having 4-transistor pixel structures with buried, gated diodes generally provide better image quality than their counterparts employing 3-transistor CMOS image sensors because the buried photodiode configuration reduces the amount of surface generated current which reduces dark current, and because the transfer gate used to access the photodiode enables the use of correlated double sampling which reduces noise. However, as described below, dark current can be generated in regions of the pixel in addition to the photodiode.
A 4-transistor pixel typically includes a photodiode, a charge-to-voltage conversion region (“floating diffusion”), and a charge transfer region. The photodiode and floating diffusion are built in a substrate (e.g. silicon) with the charge transfer region typically being a MOSFET (“transfer gate”) positioned between the diode and floating diffusion.
The pixel has two modes of operation, a charge collection or integration mode and a charge transfer or readout mode. Prior to integration, the floating diffusion is set to a “high” voltage and the transfer gate is turned on so as to extract all electrons from the photodiode so that is becomes “fully depleted.” During integration, the transfer gate is held at a low voltage (e.g. ground) and the diode is exposed to light and collects photo-generated electrons. At the conclusion of the integration period, during the readout mode, the floating diffusion is again set to a “high” voltage and the transfer gate is turned on so that electrons are transferred from the photodiode to the floating diffusion via the charge transfer region.
The charge transfer region is essentially the channel region of a “normally off” or enhancement-mode type NMOSFET. During integration, the transfer gate is “turned off.” However, even though the transfer gate is turned off, current can be generated in surface of the channel region, particularly in the region closest to the photodiode. This current contributes to dark current and is generated by sources other than incident light (e.g. heat). Because of its proximity to the photodiode, this dark current may leak into the photodiode during integration, thereby resulting in “noise” and limiting the pixel's imaging performance.
To reduce dark current generated in this region, one type of pixel structure employs a charge transfer region having a surface region extending essentially along an entire length and width of the transfer gate that is implanted with a dopant that enhances the conductivity of the charge transfer region relative to the substrate. By enhancing the conductivity in this fashion, the surface region of the charge transfer region is accumulated with “holes” when the transfer gate is held at the low-voltage level so as to quench dark current generation in this area.
Image quality can also be adversely affected by incomplete charge transfer from the photodiode to the floating diffusion and by subsurface leakage current. During the charge transfer cycle, the transfer gate voltage is set “high.” As charge is transferred from the photodiode to the floating diffusion, the potential of the floating diffusion begins to fall while the potential of the photodiode begins to rise. If the floating diffusion potential drops to the level of the transfer gate channel surface potential, some of the charge being transferred from the photodiode may remain in the transfer gate channel region until the end of the charge transfer cycle when the transfer gate voltage is dropped toward ground. At this point, some these charges (i.e. electrons) may return to the photodiode rather than be transferred to the floating diffusion. This is sometimes referred to as “spill-back” or “slosh-back.” This incomplete charge transfer can affect the amount of charge accumulated and transferred during the next integration period, producing image lag or temporal noise.
Subsurface leakage occurs during integration when electrons move from the photodiode region to the floating diffusion. Such leakage will result in an inaccurate reading of the amount of charge collected during an integration period, thereby reducing the image quality.
While the above described structure is generally effective at reducing dark current by implanting the whole of the charge transfer region with a dopant to enhance its conductivity relative to the substrate, such a structure does not address issues of subsurface leakage current and image lag due to charge spill-back.